The majority of deaf children are at risk for impoverished and/or delayed exposure to language, spoken or signed, because they have limited access to speech sounds and have parents who are not native users of a sign language. Work on the critical period has shown that delayed exposure to a first language has lasting effects on many aspects of development. The goal of this project is to understand how exposure to primarily non-native language input during the critical period affects vocabulary acquisition. We focus on phonological neighborhood density and iconicity?two factors that typically shape sign language vocabulary development (Caselli & Pyers, 2017), but may be uniquely affected by impoverished linguistic input during the critical period. Hearing parents, as second language learners of American Sign Language (ASL), are themselves affected by the age at which they begin learning ASL. A hallmark of second language signing is a tradeoff between phonology and iconicity, whereby their sign production is likely to approximate the iconic motivation of a sign (e.g., illustrate the horns of a cow) at the expense of phonological accuracy. We ask whether children who have hearing parents mimic this tradeoff, and are more sensitive to iconicity and less sensitive to phonological neighborhood density than children with native ASL exposure. We also characterize deaf and hearing parents? use of iconic signs during naturalistic interactions between parents and children, in an effort to uncover mechanisms that may make learning contexts more or less informative for highly iconic signs than less iconic signs. Among hearing children, early vocabulary is a robust predictor of later language development. We ask whether similar relationships exist between early ASL vocabulary and later language development (morphosyntactic acquisition and later vocabulary knowledge). In addition to significant advances in theories of language acquisition and the critical period, this work will have immediate applied benefits. It will result in ASL- LEX 2.0, the first early assessment of ASL that can be used widely in infants and young children to quickly identify limited ASL proficiency. All of the ASL-CDI 2.0 reports data will be made publicly available via WordBank, a cross-linguistic repository of data on children?s early vocabulary, and age of acquisition information will be integrated into ASL-LEX, an interactive online database of ASL vocabulary. Together this, project will provide a detailed view of early ASL vocabulary acquisition, and can help practitioners and researchers develop interventions to mitigate the risks and effects of impoverished language exposure during the critical period.